add hook_help

search_api_elasticsearch_synonym.module

 <?php
+
+/**
+ * @file search_api_elasticsearch_synonym.
+ */
+
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Implements hook_help().
+ */
+function search_api_elasticsearch_synonym_help($route_name, RouteMatchInterface $route_match) {
+  switch ($route_name) {
+    case 'help.page.search_api_elasticsearch_synonym':
+      $output = '';
+      $output .= '<p>' . t('<strong>Search api elasticsearch synonym</strong> This module provides synonym functionality for Elasticsearch powered by Search API.'). '</p>';
+      $output .= '<h3>' . t('<strong>Install module</strong>'). '</h3>';
+      $output .= '<p>' . t('Go to /admin/config/search/search-api-elasticsearch-synonym to configure module'). '</p>';
+      $output .= '<p>' . t('Enable checkbox, enter synonyms in Solr format, hit Save configuration button.'). '</p>';
+      $output .= '<h3>' . t('<strong>Known issues</strong>'). '</h3>';
+      $output .= '<p>' . t('Does not work when fuzziness is enabled, set fuzziness to auto, see https://github.com/elastic/elasticsearch/issues/25518.'). '</p>';
+      $output .= '<p>' . t('Does not correctly find synonyms when stemming is enabled. As a workaround enter stem as a synonym (e.g. univers , universe , cosmos.'). '</p>';
+      return $output;
+  }
+}